You really can't with O2 if you are following normal procedures. If you continuously oxygenate it's bad, but no one does that. The solubility limit of O2 in wort is fine for yeast if you are pitching the proper amount of healthy yeast. The solubility of O2 in wort is less than in water.
Can that be reconciled with the statement in the recent Zymurgy "Brew Lager like a German" article...
"It is easy to over-oxygenate the wort in a small vessel with pure O2, so tread carefully. Levels of oxygen over 12 mg/l can have a toxic effect on the yeast and result in stopped fermentation"
...haven't people been talking about pure O2 being able to get as high as 26 or so? regular air only being able to reach 8 ?